Many vehicles, such as buses and ambulances, are provided with on board electrical systems including one or more batteries for supplying electrical power to a variety of loads such as lights, specialized equipment, and vehicle accessories. Some of these loads are motors, such as a starting motor for a large diesel engine, or for driving air conditioners, which must provide a high torque and therefore have a high power requirement. It is desirable to supply such high power loads at a voltage greater than 12 volts, such as 24 volts, in order to meet the high torque requirements without excessive current and consequent resistive losses. However, storage batteries for vehicles are conventionally produced with a standard 12 volt nominal battery voltage. Furthermore, lights, fare boxes, radios, transmissions and many accessories and other equipment for use on vehicles are conventionally manufactured for use with a nominal 12 volt system.
Therefore, it is desirable to take advantage of the cost savings of using conventionally available 12 volts electrical equipment and 12 volt batteries on a vehicle while also providing a DC supply voltage and supplying greater than 12 volts to the larger loads, such as motors. This has traditionally been accomplished by utilizing two or more 12 volt batteries connected in series. The high power requirements can then be delivered at the sum voltage of the series batteries and yet power can be supplied to the 12 volt loads at the voltage of the 12 volt batteries. In this manner, the 12 volt loads can be supplied by connecting them in parallel to one of the 12 volt batteries, typically the battery which is connected to vehicle ground, and the higher voltage loads can be connected parallel to the entire set of series batteries.
Unfortunately, however, in the absence of additional circuitry, such connection causes the battery which is parallel to the 12 volt loads to become undercharged, drained, and reduced in voltage while the other battery or batteries becomes overcharged and raised to an excessive voltage. This occurs because the alternator is connected parallel to the set of series batteries and consequently all charging current used to charge the battery supplying the 12 volt loads must also flow through the other 12 volt battery or batteries. Therefore, a series battery which does not supply the 12 volt loads becomes overcharged and, as a result, the grounded battery which supplies the 12 volt loads cannot be maintained in a charged condition at full voltage.
In order to correct this problem a battery equalizer was developed and is shown in U.S. Pat. No. 4,479,083. The battery equalizer of that patent is a three terminal device which is connected to the three terminals of two series connected batteries. It causes two series connected batteries of equal voltage to contribute equally to the current supplied to the 12 volt loads as if the two batteries were connected in parallel. The battery equalizer extends battery life by maintaining the voltages of the two batteries equal because the equalizer holds the voltage, at the intermediate terminal between the two batteries, at a voltage which is equal to one-half the sum of the two battery voltages. Therefore, neither battery becomes significantly overcharged or undercharged. The equalizer of that patent, like the equalizer of the present invention may also be applied to more than two series connected batteries by applying the principles explained in the above cited patent and in this patent specification to provide a four or more terminal device.
Although the equalizer of the above cited patent was a substantial improvement, some problems remain and are discussed more fully in the following Detailed Description. For example, it was recognized that disconnection of either the grounded battery terminal or the 24 volt battery terminal can damage the battery equalizer circuit. Therefore, protective components were inserted to protect the equalizer circuit from those fault conditions. However, those protective components consume significant energy and therefore reduced the overall efficiency of the equalizer circuit. It is therefore an object and feature of the present invention to provide an equalizer circuit which will not be damaged from disconnection of the ground terminal of the grounded battery or the 24 volt terminal of the other battery and yet is protected by circuit elements which consume less energy so that the equalizer can have a higher efficiency.
Additionally, since a three terminal equalizer is used for two series connected batteries, there are several possible misconnections of the device which can occur as a result of human error. If the device of the prior art is misconnected it can be damaged by resulting electrical currents. Therefore, it is another object and feature of the present invention to provide a circuit which can be misconnected and yet will not be damaged,
In the equalizer circuit of the above prior art patent, voltage dividing resistors were used to develop a voltage which represented the average of the sum of the two battery voltages to provide a reference voltage. That reference voltage was differenced with the output voltage of the battery supplying the 12 volt loads and that difference voltage provided an error signal. The error signal was applied to a switching control circuit having a pulse width modulator to control the width of a rectangular wave applied to switch the two switching transistors of that circuit.
The problem with that system for detecting an error signal is that the circuit requires precision resistors which not only needlessly consume energy, but more importantly any variation in the resistance of those precision resistors, as a result of aging, causes corresponding errors in the developing error signal and a consequent error in the equalization of the battery voltages.
There is therefore a need for a more accurate and more stable circuit and method for detecting any equalization errors and therefore for maintaining more accurate equalization of the batteries.
Additionally, there is a need for a battery voltage equalizer which is bidirectional, that is which has the capability of transferring energy from whichever battery is at the greater voltage to the other battery at the lower voltage. Additionally, it is advantageous to provide an equalizer which requires no circuit breaker or fuse.